The embodiment of the invention discloses a gas discharge tube and an overvoltage protection device. The gas discharge tube comprises a sealing discharge cavity and at least two conductive electrodes,wherein the inner portion of the sealing discharge cavity is filled with discharge gas; discharge electrode faces of the at least two conductive electrodes are located on the inner surface and/or inner portion of the sealing discharge cavity; the sealing discharge cavity comprises at least one telescopic part, and the at least one telescopic part seals partial regions of the sealing discharge cavity; the at least one telescopic part is used for discharging between the at least two conductive electrodes, so that when the discharge gas is heated to expand, the at least one telescopic part stretches or retracts to enlarge a discharging gap between the discharge electrode faces of the at least two conductive electrodes, and so that the follow current of the gas discharge tube is blocked. According to the technical scheme of the gas discharge tube and the overvoltage protection device, effective overvoltage protection can be realized, and after the overvoltage disappears, the follow current flowing through the gas discharge tube can be automatically and effectively blocked.